To create a liquid crystal display (LCD), one starts with two pieces of polarized glass. On the side of the glass that does not have the polarizing film on it there is a special polymer that creates microscopic grooves in the surface. The grooves are in the same direction as the polarizing film. On one of the filters, a coating of nematic liquid crystals is added. The grooves will cause the first layer of molecules to align with the filter's orientation. Then the second piece of glass with the polarizing film is added at a right angle to the first piece. Each successive layer of twisted nematic (TN) molecules will gradually twist until the uppermost layer is at a right angle to the bottom, matching the polarized glass filters.
As light strikes the first filter, it is polarized. The molecules in each layer then guide the light they receive to the next layer. As the light passes through the liquid crystal layers, the molecules also change the light's plane of vibration to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If the final layer is matched up with the second polarized glass filter, then the light will pass through.
If an electric charge is applied to liquid crystal molecules, the molecules untwist. When they straighten out, they change the angle of the light passing through them so that it no longer matches the angle of the top polarizing filter. Consequently, no light can pass through that area of the LCD, which makes that area darker than the surrounding areas.
A basic LCD is constructed in layers. It has a mirror in back, which makes it reflective. Then, next is added a piece of glass with a polarizing film on the bottom side and a common electrode plane made of indium-tin oxide on top. A common electrode plane covers the entire area of the LCD. Above that is the layer of liquid crystal substance. Next comes another piece of glass with an electrode in the shape of the rectangle on the bottom and, on top, another polarizing film, at a right angle to the first one.
The electrode is hooked up to a power source like a battery. When there is no current, light entering through the front of the LCD will simply hit the mirror and bounce right back out. But when the battery supplies current to the electrodes, the liquid crystals between the common-plane electrode and the electrode shaped like a rectangle untwist and block the light in that region from passing through. That makes the LCD show the rectangle as a black area.
An LCD that can show colors typically have three subpixels with red, green and blue color filters to create each color pixel. Through the careful control and variation of the voltage applied, the intensity of each subpixel can range over multiple shades (e.g., 256 shades). Combining the subpixels produces a possible palette of many more (e.g., 16.8 million colors (256 shades of red×256 shades of green×256 shades of blue)).
LCD technology is constantly evolving. LCDs today employ several variations of liquid crystal technology, including super twisted nematics (STN), dual scan twisted nematics (DSTN), ferroelectric liquid crystal (FLC) and surface stabilized ferroelectric liquid crystal (SSFLC).
Along the edge of a typical LCD display or television is a cold cathode fluorescent (CCFL) or an array of light-emitting diodes (LEDs). Using the optical system, these lights backlight the pixels of the display. Indeed, these lights are typically the only lights in the display.
The optical system includes a first sheet that makes a nice even white background for the light. The next piece is called a “light-guide plate” (LGP). When light enters from the edge of the LGP, it propagates through the plate by total internal reflection, unless it hits one of many dots. The dots make some of the light rays emerge out the front. Then engineers place a diffuser film; it helps eliminate the dot pattern from the light-guide plate. Then comes a “prism film.”
This is used because light from the backlight emerges not only perpendicular to the back surface, but also at oblique angles. This sheet will increases the perpendicular part a bit: Another diffuser film is added to form an evenly lit surface.
In a typical LCD display, the “backlight” is always on when the device is on, but what controls what is seen is a piece of glass: It functions as a shutter. At the back and front of this glass sheet are two polarizers. They stick tightly to the piece of glass.
The glass has two panes separated by tiny glass beads to keep them separated and with organic molecules known as liquid crystals. These crystals have interesting properties in that they do not allow light to pass uniformly along both axes. Grooves are formed on the surface of both pieces of glass at 90 degrees to one another. The molecules in-between line up in a beautiful helix.
When light from the backlight passes through the first polarizer and enters the sandwich, it's rotated by the liquid crystals so as to allow it to pass through the second polarizer and emerge out the front of the screen. This is known as the normally white mode. Applying an electric field across the sandwich causes the crystals to line up lengthwise.
Now the light that passes through the first polarizer is not rotated by the crystals and can no longer pass through the front of the screen. This is the normally black mode.
By controlling the voltage between these transparent electrodes, the intensity of the light that passes through can be controlled. Each pixel includes red, green, and blue sections. These are sub-pixels: The three together make a single pixel.
In the sandwich these are simply colored tiles that overlay the front transparent electrodes. They follows the RGB color model: The “electrode-shutter” behind the sub-pixels is adjusted so that they make up a particular color. For example, to get the color of the blue in my shirt we set the red sub-pixel to 12% of maximum intensity, green to 21% and blue to about 50%.
On the back pane engineers paint tiny devices called thin film transistors (TFT). Each sub-pixel has transistor which controls it. This transistor functions as a switch that allows the screen to be updated row by row.
By applying a voltage to a specific row while keeping the other rows grounded we allow each sub-pixel in that row to receive video data coming from the top of the screen. Only one row can receive information at a time, but the speed with which this happens for each row is so fast that your brain blends it into a fluid image.
The Detailed Description references the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components.